Recently, a technique has become popular for operating a semiconductor device in a stable condition in a wide power-supply voltage range by changing the internal circuit operation in accordance with the power-supply voltage value. For this reason, a voltage detection circuit for detecting a power-supply voltage value has become important.
A conventional voltage detection circuit will be explained hereinafter with reference to FIGS. 23-25. FIG. 23 shows the construction of the conventional voltage detection circuit. FIG. 24 shows the relationship between the power-supply voltage and the output voltage signal in the conventional voltage detection circuit. FIG. 25 shows the relationship between the power-supply voltage and the current drain.
Firstly, the circuit construction will be explained. As shown in FIG. 23, the Qp 61 is a P-channel type MOS transistor whose source is connected with the power-supply voltage VDD and whose gate and train are connected with the node N 61. The Qp 62 is a P-channel type MOS transistor whose source is connected with the node N 61 and whose gate and train are connected with the node N 62. The Qp 63 is a P-channel type MOS transistor whose source is connected with the node N 62 and whose gate and train are connected with the node N 63. The Qn 61 is an N-channel type MOS transistor whose source is connected with the ground voltage VSS, whose gate is connected with the power-supply voltage VDD, and whose train is connected with the node N 63. The Qp 64 is a P-channel type MOS transistor and the Qn 62 is an N-channel type MOS transistor which compose a first NOT circuit 61. The source, gate, and drain of the P-channel type MOS transistor Qp 64 are connected with the ground voltage VDD, the node N 63, and the node N 64, respectively. The source, gate, and drain of the N-channel type MOS transistor Qn 62 are connected with the ground voltage VSS, the node N 63, and the node N 64, respectively. The node N 64 is connected with the input terminal of a second NOT circuit 62. The second NOT circuit 62 is applied with the voltage detection signal VDT 60 from the node N 64, and generates the output voltage signal VOUT 60.
The operation of the voltage detection circuit will be explained as follows. As shown in FIG. 24, the logical voltage of the output voltage signal VOUT 60 which is obtained at the output terminal of the second NOT circuit 62 becomes “L” when the power-supply voltage VDD is less than 4V and becomes “H” when the voltage VDD is about 4V or higher under predetermined conditions.
This result is due to the following ground. The electric potential of the node N 63 is lower than the power-supply voltage VDD by the voltage drop of the P-channel type MOS transistors Qp 61-Qp 63. The electric potential becomes 2V, for example.
On the other hand, the threshold level of the first NOT circuit 61 which is composed of the P-channel type MOS transistor Qp64 and the N-channel type MOS transistor Qn 62 is about ½ of the power-supply voltage VDD. Therefore, when the power-supply voltage VDD is about 4V, the electric potential of the node N 64 which is connected with the input terminal of the first NOT circuit 61 becomes about 2V, so that the logical voltage of the node N 64, or the voltage detection signal VDT goes from “H” to “L”, and the logical voltage of the output voltage signal VOUT 60 which is the output of the second NOT circuit 62 goes from “L” to “E”.
The current drain of the voltage detection circuit will be explained as follows. As shown in FIG. 24, when the power-supply voltage VDD is about 4V, the node N 63 which is the input terminal of the first NOT circuit 61 consisting of the P-channel type MOS transistor Qp 64 and the N-channel type MOS transistor Qn 62 has an intermediate electric potential between the power-supply voltage VDD and the ground voltage VSS. Consequently, both the P-channel type MOS transistor Qp 64 and the N-channel type MOS transistor Qn 62 are in the on state, that is, the first NOT circuit 61 temporarily falls into the short-circuit state. The current drain In 60 which runs through the N-channel type MOS transistor Qn 62 has a peak of 0.6 μA or so. Even when the power-supply voltage VDD is not about 4V, the current drain In 60 is 0.1 μA or higher as shown in FIG. 25.
However, in the conventional voltage detection circuit, when the electric potential of the node N 63 which is the input of the first NOT circuit 61 has an intermediate electric potential between the power-supply voltage VDD and the ground voltage VSS, both the P-channel type MOS transistor Qp 64 and the N-channel type MOS transistor Qn 62 become the on state, that is, fall into temporary short-circuit state, which leads to an increase in the current drain. The current drain for the entire voltage detection circuit is large in other states, too.
In view of these problems, the object of the present invention is to provide a voltage detection circuit which reduces the peak of the current drain in the temporary short-circuit state and decreases the current drain as the entire circuit.
On the other hand, when a predetermined voltage is detected by the voltage detection circuit, a power-on/off reset circuit for immediately suspending the operations of the devices such as a logic circuit or a memory circuit might destroy memory data in the memory circuit when the operation is immediately suspended. Although there is no problem in the logic circuit, the memory circuit needs data re-writing (restore or refresh) after a readout. For this reason, it is difficult to properly terminate a sequence in operation.
In view of these problems, another object of the present invention is to provide a power-on/off reset circuit which properly terminates a sequence in operation.